RFC 7869
The "vnc" URI Scheme
Pages: 25
Informational
Informational
Independent Submission D. Warden Request for Comments: 7869 Dell Products LP Category: Informational I. Iordanov ISSN: 2070-1721 Undatech May 2016 The "vnc" URI SchemeAbstract
Virtual Network Computing (VNC) software provides remote desktop functionality. This document describes a Uniform Resource Identifier (URI) scheme enabling the launch of VNC clients from other applications. The scheme specifies parameters useful in securely connecting clients with remote hosts. Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This is a contribution to the RFC Series, independently of any other RFC stream. The RFC Editor has chosen to publish this document at its discretion and makes no statement about its value for implementation or deployment. Documents approved for publication by the RFC Editor are not a candidate for any level of Internet Standard; see Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7869. Copyright Notice Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document.
Table of Contents
1. Introduction ....................................................3 1.1. Requirements Language ......................................3 2. The "vnc" URI Scheme ............................................3 2.1. URI Scheme Syntax ..........................................3 2.1.1. URI Parameters ......................................4 2.1.2. Data Types ..........................................9 2.2. Processing URIs ...........................................11 2.2.1. Error Handling .....................................12 2.2.2. Connection Profile Matching ........................12 2.3. Connection Channel Types ..................................12 2.3.1. The "Integrated SSH" Channel Type ..................12 2.3.2. The "Secure Tunnel" Channel Type ...................14 3. Security Considerations ........................................15 3.1. Application Trust .........................................16 3.2. URI Handling ..............................................16 3.3. Host Identification .......................................17 3.4. Connection Database Integrity .............................18 4. IANA Considerations ............................................18 4.1. "vnc" Scheme ..............................................18 4.2. Remote Framebuffer Security Types .........................18 4.3. VNC URI Group .............................................19 4.4. VNC URI Connection Channel Types ..........................19 4.5. VNC URI ID Hash Algorithms ................................19 4.6. VNC URI Parameters ........................................21 5. References .....................................................22 5.1. Normative References ......................................22 5.2. Informative References ....................................23 Appendix A. "vnc" URI Template ....................................24 Acknowledgments ...................................................25 Authors' Addresses ................................................25
1. Introduction
Virtual Network Computing (VNC) clients are used to support remote desktop connectivity based on the Remote Framebuffer (RFB) Protocol [RFC6143]. It is often desirable to integrate such functionality with other software. However, the lack of a standard method for specifying VNC client parameters has limited such integration. The "vnc" Uniform Resource Identifier (URI) scheme specified in this document facilitates the launch of VNC clients from applications in browser-based, desktop, and mobile environments. Using this scheme, users and application vendors will be able to integrate remote desktop capabilities without being tied to a particular client. Remote desktop clients often store connection profiles in a local connection database. By associating connections specified in a URI with those stored in a database, client-specific options can be automatically applied to a connection launched from another application, even when that application is unaware of those options. Connections to VNC servers are often secured using mechanisms including Transport Layer Security / Secure Sockets Layer (TLS/SSL) tunneling [RFC5246] and Secure Shell (SSH) [RFC4251] tunneling, which are outside the scope of the RFB protocol. Defining the behavior of these client-integrated security options enables their use with "vnc" URIs.1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. In this document, these words will appear with that interpretation only when in ALL CAPS. Lowercase uses of these words are not to be interpreted as carrying the significance described in RFC 2119.2. The "vnc" URI Scheme
2.1. URI Scheme Syntax
The normative syntax of the "vnc" URI is defined in the <vnc-uri> rule in the following syntax specification. This specification uses the Augmented Backus-Naur Form (ABNF) as described in [RFC5234]. The "vnc" URI conforms to the generic URI syntax specified in [RFC3986]. The <userinfo>, <host>, <port>, <unreserved>, and <pct-encoded> rules are defined in [RFC3986].
vnc-uri = "vnc://" [ userinfo "@" ] [ host [ ":" port ] ] [ "?" [ vnc-params ] ] vnc-params = param "=" value *("&" param "=" value) ["&"] param = 1*( param-char ) value = *( param-char ) param-char = unreserved / pct-encoded / unreserved-symbols unreserved-symbols = ":" / "/" / "@" / "!" / "$" / "'" / "(" / ")" / "*" / "," / ";" The "?", "=", and "&" characters are used to delimit VNC parameters and must be percent-encoded when representing a data octet as specified in [RFC3986]. Within the <vnc-params> portion of a "vnc" URI, the <unreserved-symbols> do not have special meaning and need not be percent-encoded when representing a data octet. A "vnc" URI has the general form: vnc://host:port?param1=value1¶m2=value2... The host information and each parameter value specify information used in establishing or operating the remote desktop session as specified in Section 2.1.1. For example: vnc://10.0.0.1:5901?VncPassword=secret&SecurityType=2 This example indicates a VNC connection to the host at IP "10.0.0.1" on port "5901" with VNC password "secret" using the VNC Authentication security type.2.1.1. URI Parameters
A description of host information and URI parameters is provided in this section. Information on the constraints of various data types is provided in Section 2.1.2. All parameters are considered optional; however, a client will not be able to connect without sufficient information. A parameter without a specified default value indicates that no default value is implied by this URI scheme; however, VNC clients can apply implementation-dependent default behaviors otherwise consistent with this document.
The <userinfo> value is deprecated and processed only in an
implementation-specific manner. The <userinfo> component MUST NOT be
generated in an environment where a client supporting an updated URI
format is expected to be available. When processing a URI value from
an untrusted source, VNC clients SHOULD alert the user in order to
mitigate the risk that the URI is constructed to obscure the identity
of the remote host unless the URI can be validated or backwards-
compatibility considerations make an alert impractical.
The <host> and <port> values in the "vnc" URI specify the address of
the VNC server on the remote host:
+------------+------------+-----------------------------+----------+
| Name | Type | Description | Default |
+------------+------------+-----------------------------+----------+
| host | string | VNC server hostname or IP | none |
+------------+------------+-----------------------------+----------+
| port | ushort | VNC server port | 5900 |
+------------+------------+-----------------------------+----------+
The "vnc" URI parameter values specify remote desktop connection or session properties, including aspects of client operation, usability, and security as specified in the table below: +---------------+---------+-----------------------------+----------+ | Name | Type | Description | Default | +---------------+---------+-----------------------------+----------+ |ConnectionName | string | Name of connection profile | none | +---------------+---------+-----------------------------+----------+ |VncUsername | string | VNC server username | none | +---------------+---------+-----------------------------+----------+ |VncPassword | string | VNC server password | none | +---------------+---------+-----------------------------+----------+ |SecurityType | enum | RFB security type used | none | | | <rfbsec>| | | +---------------+---------+-----------------------------+----------+ |ChannelType | enum | Connection channel type | none | | | <chan> | | | +---------------+---------+-----------------------------+----------+ |SshHost | string | SSH server hostname or IP | <host> | +---------------+---------+-----------------------------+----------+ |SshPort | ushort | SSH server port | 22 | +---------------+---------+-----------------------------+----------+ |SshUsername | string | SSH username | none | +---------------+---------+-----------------------------+----------+ |SshPassword | string | SSH password | none | +---------------+---------+-----------------------------+----------+ |IdHashAlgorithm| enum | Hash algorithm used with | none | | | <idhash>| "IdHash" parameter | | +---------------+---------+-----------------------------+----------+ |IdHash | string | Expected hash of remote | none | | | <hex> | public key or certificate | | +---------------+---------+-----------------------------+----------+ |ColorLevel | enum | Client color depth/mode | none | | | <clevel>| | | +---------------+---------+-----------------------------+----------+ |ViewOnly | boolean | Client is view only | false | +---------------+---------+-----------------------------+----------+ |SaveConnection | boolean | Store connection info | none | +---------------+---------+-----------------------------+----------+ o ConnectionName, SaveConnection "ConnectionName" is used to identify a connection profile in both the launching application and VNC client. Profiles are applied as described in Section 2.2.2. If omitted, the client MAY generate a name based on the host, port, and/or other parameters. The VNC client MAY normalize the name as required.
If true, "SaveConnection" indicates a connection profile should be
created or updated and stored in the client connection database.
If false, no profile should be updated or persisted.
o VncUsername, VncPassword, SecurityType
The "SecurityType" parameter indicates which RFB security type
applies to the connection. RFB security types are recorded in the
IANA "Remote Framebuffer Security Types" registry created by
[RFC6143]. The VNC client will use this information to determine
which parameters are required and establish the connection.
VNC clients can sometimes automatically negotiate a security type
with a server. Specifying the security type controls the security
negotiation. Specifying the security type also allows a client to
prompt for necessary security parameters prior to establishing a
connection. Parameters may take time to enter on mobile clients
and could otherwise result in timeouts and/or security lockouts.
If the specified type is not supported by the server, an error
SHOULD be indicated as described in Section 2.2.1.
"VncUsername" and "VncPassword" are used when applicable to
authenticate to the VNC server using the specified "SecurityType".
Since passwords often contain arbitrary characters, they will
often require percent encoding.
o ChannelType
"ChannelType" specifies the transport stream used to carry
connection data. This allows a client to initiate a connection
using a secure transport protocol such as SSH prior to connecting
to the VNC server socket. Use of this value in the context of the
"Integrated SSH" and "Secure Tunnel" channel types is provided in
Section 2.3.
o SshHost, SshPort, SshUsername, SshPassword
The SSH parameters are intended for use with the "Integrated SSH"
channel type described in Section 2.3.1. These parameters can
also be used with any future SSH-based channel types. Since
passwords often contain arbitrary characters, they will often
require percent encoding.
o IdHashAlgorithm, IdHash
The "IdHashAlgorithm" and "IdHash" values are used to verify the
expected identity of the remote system based on its public key or
certificate. Use of these values in the context of the
"Integrated SSH" and "Secure Tunnel" channel types is provided in
Section 2.3.
o ColorLevel
The "ColorLevel" parameter specifies the color model to use for
data transfer and display as specified in Section 2.1.2. If the
requested color model is unsupported, the behavior is
implementation dependent.
o ViewOnly
If "ViewOnly" is true, the VNC client SHOULD operate in a display-
only mode and refrain from sending input data including KeyEvent,
PointerEvent, and ClientCutText messages specified in Section 7.5
of [RFC6143] unless this mode is unsupported by the client.
Parameter names SHOULD be provided in the case specified in this
document; however, for compatibility, clients SHOULD accept
parameters in a case-insensitive manner. Values SHALL be interpreted
in a case-sensitive manner, unless otherwise noted.
Additional parameters likely to be useful with multiple VNC clients
can be added to the "VNC URI Parameters" registry as specified in
Section 4.6 of this document. Individual clients MAY support
parameters specific to that client. VNC clients supporting
application-specific parameters SHOULD include a distinguishing
prefix within the parameter name, such as the name of the application
package specified in source code except when precluded by
compatibility constraints. For example:
vnc://?com.dell.vncclient.ScreenMode=2&
It can also be expected that clients will maintain backward
compatibility with legacy URI formats and parameters.
Legacy software applications respond to "vnc" URIs in different ways
and may fail to behave as expected. It is advisable to test "vnc"
URIs with specific applications or consult application-specific
documentation.
2.1.2. Data Types
"vnc" URIs can be percent-encoded as specified in [RFC3986] and MUST be decoded. After decoding, the following type constraints and semantics apply: o string Values of "string" type are UTF-encoded strings as specified in [RFC3629]. The "string<hex>" subtype used in the "IdHash" consists of colon- delimited ":" octets displayed in hexadecimal. For example: 5D:D2:39:57 Comparison of "string<hex>" values SHALL be case insensitive; however, the uppercase notation is preferred for readability. o enum The "enum" types consist of specific enumerated subtypes and are represented by their decimal value. The "enum<rfbsec>" values represent an RFB security type included in the IANA "Remote Framebuffer Security Types" registry created by [RFC6143]. "enum<chan>" values represent connection channel types listed in the "VNC URI Connection Channel Types" registry created by Section 4.4 of this document. Initial values are: Value Description -------- -------------- 1 Standard TCP 23 Secure Tunnel 24 Integrated SSH The "Standard TCP" channel type represents a generic TCP connection. The "Secure Tunnel" and "Integrated SSH" [RFC4252] channel types are described in Section 2.3.
Values of the "enum<idhash>" parameter represent secure hash
algorithms in the "VNC URI Hash Algorithms" registry created by
Section 4.5 of this document. The initial values include:
Value Description
-------- ------------
1 MD5
2 SHA1
4 SHA256
The MD5 algorithm is described in [RFC1321]. The SHA-1 and
SHA-256 algorithms are described in [SHS].
Values of the "enum<clevel>" subtype represent a color level. In
the table below, the columns have the meaning specified in
Section 7.4 of [RFC6143]:
BPP = bits-per-pixel
TC = true-color-flag
RM = red-max
GM = green-max
BM = blue-max
RS = red-shift
GS = green-shift
BS = blue-shift
The values are:
Value Description BPP Depth TC RM GM BM RS GS BS
----- --------------- --- ----- -- ---- ---- ---- -- -- --
1 Black and White 8 3 t 1 1 1 2 1 0
2 Grayscale 8 6 t 3 3 3 4 2 0
3 8 Colors 8 3 t 1 1 1 2 1 0
4 64 Colors 8 6 t 3 3 3 4 2 0
5 256 Colors 8 8 t 7 7 3 0 3 6
6 16-bit Color 16 16 t 31 63 31 11 5 0
7 24-bit Color 32 24 t 255 255 255 16 8 0
8 30-bit Color 32 30 t 1023 1023 1023 0 10 20
A value of "t" indicates the true-color-flag should be set. The
big-endian-flag (see Section 7.4 of [RFC6143]) should be set as
required for the system.
o ushort
The "ushort" values represent unsigned 16-bit integers expressed
in decimal digits with value between 0-65535 inclusive.
o boolean
"boolean" values represent conditions that are true or false and
are represented as either "true" or "false" respectively. For
maximum compatibility, clients SHOULD accept the value 1 as
representing true values and 0 as representing false values.
Clients SHOULD perform parsing of "boolean" values in a case-
insensitive manner.
An example "vnc" URI including several of these data types is:
vnc://localhost:5900?ConnectionName=Server&SecurityType=2&
IdHash=0D:3A:72:08:57:EA:4D:30&SaveConnection=false&
Note the above example should be considered to be a contiguous
string without line breaks or whitespace and is broken into
multiple lines in this document for readability.
2.2. Processing URIs
Conceptually, a "vnc" URI supports only a "VIEW" operation,
indicating the user wishes to view the remote desktop accessible via
the URI reference.
In general, when a VNC client receives a "vnc" URI, it will initiate
a remote desktop connection with the RFB protocol using the specified
host information and parameter values. Initiating the connection
using a connection channel mechanism such as those specified in
Section 2.3 might require processing prior to establishing the RFB
connection. A client MAY attempt to automatically discover or
negotiate appropriate connection channel, security, or other
parameter values.
The process for negotiating security types is specified in [RFC6143].
Supported connection channels could be discovered by testing channel
types to detect when a channel is successfully established. To best
integrate with other applications, the VNC client SHOULD initiate the
connection with minimal or no user intervention, whenever sufficient
information is available and adequate security is preserved.
Host information and parameter values may be provided through
connection profiles. When a parameter value is not available from
either a URI or a connection profile described in Section 2.2.2, the
default value specified in Section 2.1.1 SHOULD be applied. If
available parameters are not sufficient to establish a connection,
the VNC client SHOULD present a session initiation data-entry screen.
2.2.1. Error Handling
In a typical interactive environment, if an error prevents a session from being established, the VNC client presents an error message to the user. When the message is acknowledged, the console application can show a session initiation data-entry screen populated with available session parameters, or it can terminate. If an error occurs after a session is successfully established that terminates the connection, the VNC client presents a termination notification to the user. When the termination notification is acknowledged, the client can present a reconnection prompt or terminate. When an error occurs in a dedicated environment (such as a kiosk system), the system can transmit an alert to the remote operator, record a log entry, and execute appropriate fallback behavior such as automatically attempting to reestablish a session or displaying a generic message requesting servicing.2.2.2. Connection Profile Matching
VNC clients MAY store remote desktop session settings in connection profiles. If the client is able to uniquely identify and associate a connection request with a connection profile based on the "ConnectionName" parameter value, remote host IP address, or hostname / fully qualified domain name, the VNC client SHOULD apply profile values for those settings that do not have values supplied in the "vnc" URI. When profile data is unavailable, the VNC client MAY apply global application defaults for settings not supplied in the URI and for which the scheme does not specify a default value. The VNC client MUST NOT override supplied parameters with profile values or global defaults. When the "SaveConnection" parameter value is true, within the VNC client, a connection profile SHOULD be created or updated with the values supplied in the "vnc" URI. Profile updates and storage should be consistent with the recommendations in Section 3.4.2.3. Connection Channel Types
2.3.1. The "Integrated SSH" Channel Type
The "Integrated SSH" channel type establishes an SSH connection to a host, authenticates with SSH password authentication, establishes a secure tunnel to the VNC host/port, and then connects to the VNC server using a supported "SecurityType". The secure tunnel will provide encryption and data integrity, while verifying the public key authenticates the server. The SSH architecture is specified in [RFC4251]. The steps are detailed below:
1. The VNC client initiates a transport-level connection to the
"SshHost" on the "SshPort" specified in the parameter values with
a key exchange as described in [RFC4253].
2. When the VNC client receives the server key (or certificate), the
hash of the key (or certificate) is computed using the algorithm
corresponding to the "IdHashAlgorithm" parameter value and
compared with the expected "IdHash" value (if available). If the
certificate hash cannot be verified, the client alerts the user or
operator. In a typical interactive environment, the alert
provides the remote system's identifying information including the
hash value and allows the user to terminate the connection. The
alert could allow the user to accept the key and continue
establishing the connection. In a dedicated environment (such as
a kiosk system), the system can transmit an alert to the remote
operator, record a log entry, and execute appropriate fallback
behavior such as displaying a generic message requesting
servicing.
3. The SSH client authenticates the user using the "SshUsername" and
"SshPassword" parameter values according to the "password"
authentication mechanism described in [RFC4252].
4. The SSH client opens a TCP/IP channel as specified in [RFC4254]
from the local system to the system indicated by the <host> and
<port> information values.
5. The VNC client establishes an RFB connection to the VNC server
over the channel and authenticates using the "SecurityType" as
described in [RFC6143] or other reference.
The VNC client MAY establish the connection described in this section
using an external SSH client, by launching the client and then
connecting to a secure tunnel created between a local port and the
VNC server.
If the VNC client is supplied with additional parameters outside the
scope of this document, it MAY perform a variation of these steps
consistent with the underlying protocols, for example, by using
"publickey" SSH client authentication [RFC4252] or providing another
form of authentication to the VNC server. The specific negotiation
of SSH parameters such as cipher suite configuration is outside the
scope of this document.
Many SSH clients present key hashes using MD5, and it can be expected
that launching applications will specify the hash be displayed in the
manner its users are familiar with.
For compatibility, when the "SecurityType" parameter value is "Integrated SSH" (24), a VNC client MUST treat the value as a request to use "Integrated SSH" as the "ChannelType". However, this value SHOULD NOT be supplied for the "SecurityType" parameter unless required for backward compatibility as the channel is established prior to connecting to the server and is not consistent with the negotiation of other security types.2.3.2. The "Secure Tunnel" Channel Type
The "Secure Tunnel" channel type establishes a TLS connection with a remote server using certificate authentication, over which a connection to the VNC server is established using a supported "SecurityType". The secure tunnel will provide encryption and data integrity, while verifying the certificate authenticates the server. The TLS protocol is specified in [RFC5246]. The steps are detailed below: 1. The VNC client initiates the TLS Handshake Protocol with a system indicated by the <host> and <port> information values. 2. When the server certificate is received, the hash of the key certificate is computed using the algorithm corresponding to the "IdHashAlgorithm" parameter value and compared with the expected "IdHash" value (if available). If the certificate hash cannot be verified, the client alerts the user or operator. In a typical interactive environment, the alert provides the remote system's identifying information and allows the user to terminate the connection. The alert could allow the user to accept the key and continue establishing the connection. In a dedicated environment (such as a kiosk system), the system can transmit an alert to the remote operator, record a log entry, and execute appropriate fallback behavior such as displaying a generic message requesting servicing. When providing identifying information of a host identified by an X.509 certificate [RFC5280] [X.509], the certificate subject, issuer, validity period, and certificate hash is typically included. The VNC client MAY verify the validity of the certificate. If the validity of a certificate is not confirmed, the alert includes a statement indicating such information has not been verified. 3. The client finishes establishing the TLS tunnel. 4. The VNC client establishes an RFB connection to the VNC server over the channel and authenticates using the "SecurityType" as described in [RFC6143] or other reference.
If the VNC client is supplied with additional parameters, it MAY perform a variation of these steps consistent with the underlying protocols, for example, by providing another form of authentication to the VNC server. The negotiation of specific TLS parameters such as cipher suite configuration is outside the scope of this document. The TLS protocol provides backwards compatibility with SSLv3; however, due to known security flaws, it SHOULD NOT be used. For compatibility, when the "SecurityType" parameter value is "Secure Tunnel" (23), a VNC client MUST treat the value as a request to use "Secure Tunnel" as the "ChannelType". However, this value SHOULD NOT be supplied for the "SecurityType" parameter unless required for backward compatibility as the channel must be established prior to connecting to the server and is not consistent with the negotiation of other security types.3. Security Considerations
General security concerns involving URI schemes are discussed in [RFC3986]. In implementing support for the "vnc" URI scheme, areas for particular consideration include application trust, URI handling, host identification, and connection database security. Remote desktop connectivity requires the transmission of security credentials, which could be included in a URI. If those credentials are not kept secure, an attacker can gain access to any systems using those credentials. Host addresses and connection parameters might also be considered sensitive, as such information can be used in planning an attack. URIs can also contain host identification information. It is important to securely identify the remote host system to which a connection is established. If a user connects to an attacker's system, user data, including credentials, can be exposed. Note that the RFB protocol itself may not encrypt data. To protect data in transit, RFB should be tunneled over TLS [RFC5246], SSH [RFC4251], or another secure protocol. Some VNC systems can be used without authentication. To protect the remote host, strong passwords or other authentication mechanisms need to be used.
3.1. Application Trust
A malicious application receiving VNC credentials via URI or other means can obviously misuse those credentials. To protect against this, users should only install applications from trusted sources. The integrity of application packages can be verified through digital signatures. Applications launching VNC clients can elect to launch only particular trusted clients and can specify those clients through platform-specific mechanisms. Package integrity can be verified programmatically by querying the package manager for digital signatures or other platform-specific means. The risk to a VNC client from a launching application is generally much lower, since the launching application will not receive credentials or data from the client. A VNC client can verify its caller thorough platform-specific means. VNC clients ought not to accept potentially destructive parameters from untrusted launching applications without explicit user confirmation. For example, a client-specific parameter that runs an arbitrary command upon establishing an SSH connection used for VNC tunneling is potentially destructive and high risk.3.2. URI Handling
Within a mobile or desktop environment, application launch will typically involve in-memory URI data transmission facilitated and secured by the operating system. When "vnc" URIs are exchanged or used within a system, their contents might be exposed by process listings or other instrumentation. Users need to avoid including sensitive information in "vnc" URIs that could be exposed to unauthorized observation. If sensitive URI information is exchanged across a network, for example, by providing a list of connection URIs in a web page, the data needs to be encrypted in transit and only be accessible to authorized users. When an application detects potentially sensitive information in a "vnc" URI, it needs to be handled securely or discarded. In particular, URI data on persistent storage needs to be encrypted as described in Section 3.4.
Since "vnc" URIs may contain sensitive information, applications should avoid logging the URIs even when errors occur. Users need to avoid including sensitive information in "vnc" URIs that are used with applications where logging is unavoidable. Applications that process URIs in a generic way, such as web browsers, might not detect that sensitive information is contained in a URI and could cache or store that information insecurely. It is advisable to avoid including credentials and other sensitive information in URIs that are likely to be processed in a generic way unless such caching and storage is disabled or otherwise secured.3.3. Host Identification
In the absence of verifiable host identification, a VNC client application is vulnerable to spoofing and man-in-the-middle attacks that capture VNC or host OS credentials and user data. To prevent such attacks, administrators SHOULD secure their VNC communications with TLS [RFC5246] or SSH [RFC4251] tunnels or other connection mechanisms identifying remote hosts via certificate or public key. VNC clients MUST verify the respective certificates or public keys to confirm the remote host's identity. An application launching a VNC client via URI MAY provide a certificate hash or public key hash identifying the remote host. VNC clients maintaining a connection database can also store certificate or public key data suitable for validating a host's identity. If connecting to a system identified by certificate or public key and a remote system ID hash cannot be matched to available identifying data, the VNC client needs to alert the user or operator. In a typical interactive environment, the alert will provide the remote system's identifying information and allow the user to terminate the connection. The alert can allow the user to accept the information and continue establishing the connection. In a dedicated environment (such as a kiosk system), the system can transmit an alert to the remote operator, record a log entry, and execute appropriate fallback behavior such as displaying a generic message requesting servicing. When providing identifying information of a host identified by an X.509 certificate [RFC5280] [X.509], the certificate subject, issuer, validity period, and certificate hash need to be included. The VNC client can verify the certificate validity. If the validity of a certificate is not determined, the alert needs to include a statement indicating such information has not been verified.
Identifying information of a host identified by public key, such as the endpoint of an SSH connection using a raw key, needs to include a hash of the key.3.4. Connection Database Integrity
A VNC client application and/or launching application can maintain a connection database containing remote host information, credentials, and/or connection parameters. Applications storing credentials need to ensure they are stored in an encrypted format with a decryption process requiring user-supplied or device-specific data. If supported, it is advisable for applications to have a setting disabling storage of credentials. If available, the VNC client connection database can store certificate or public key data used to verify host identification. To prevent a malicious URI from overriding the database, if identification information in the URI conflicts with information in the database, the user or operator needs to be alerted. In a typical interactive environment, the user can be prompted to accept the new information prior to updating the database.4. IANA Considerations
The "vnc" scheme has been registered in the "Uniform Resource Identifier (URI) Schemes" registry. The "Remote Framebuffer Security Types", "VNC URI Connection Channel Types", "VNC URI ID Hash Algorithms", and "VNC URI Parameters" registries support elements of the scheme.4.1. "vnc" Scheme
IANA has added the "vnc" scheme to the "Uniform Resource Identifier (URI) Schemes" registry with description "Remote Framebuffer Protocol" and reference to this document. A registration template is provided in Appendix A. The IANA schemes registry is currently located at <http://www.iana.org/assignments/uri-schemes>.4.2. Remote Framebuffer Security Types
This document references the existing IANA "Remote Framebuffer Security Types" registry in specifying security type options. RFB security types are supported in "vnc" URIs.
Security mechanisms integrated with VNC clients might need to alter the process by which a connection is established prior to the security handshake described in Section 7.1.2 of [RFC6143]. Such mechanisms should be reflected in the "VNC URI Connection Channel Types" registry described in Section 4.4 of this document rather than the "Remote Framebuffer Security Types" registry, as their use cannot be negotiated by the mechanism specified in [RFC6143]. Exceptions can be made for backwards compatibility. IANA has updated the "Secure Tunnel" and "Integrated SSH" security types to refer to this document.4.3. VNC URI Group
IANA has created a "Virtual Network Computing (VNC) Uniform Resource Identifier (URI)" group. This group contains application-level, URI- related registries distinct from those used by the RFB protocol itself.4.4. VNC URI Connection Channel Types
IANA has created a "VNC URI Connection Channel Types" registry within the "Virtual Network Computing (VNC) Uniform Resource Identifier (URI)" group. The registry includes Value, Description, and Reference columns. The initial contents of the registry are described in this document. The values of the "Secure Tunnel" and "Integrated SSH" types are copied from the RFB Security Types registry. They are: Value Description Reference -------- --------------- -------------- 0 Reserved this document 1 Standard TCP this document 23 Secure Tunnel this document 24 Integrated SSH this document The maximum acceptable value is 2,147,483,647. Future assignments to this registry should be made through the "First Come First Served" process described in [RFC5226].4.5. VNC URI ID Hash Algorithms
IANA has created a "VNC URI ID Hash Algorithms" registry within the "Virtual Network Computing (VNC) Uniform Resource Identifier (URI)" group. The registry includes Value, Description, and Reference columns.
The initial hash algorithms specified are a subset of the algorithms contained in the "TLS HashAlgorithm Registry". The initial contents of the registry are: Value Description Reference -------- ------------ -------------- 0 Reserved this document 1 MD5 this document 2 SHA1 this document 4 SHA256 this document The maximum acceptable value is 2,147,483,647. Future assignments to this registry should be made through the "First Come First Served" process described in [RFC5226].
4.6. VNC URI Parameters
IANA has created a "VNC URI Parameters" registry within the "VNC URI" group. The initial contents are described in this document. They are: +-----------------+-----------------------------+-----------------+ | Name | Description | Reference | +-----------------+-----------------------------+-----------------+ | ConnectionName | Name of connection profile | this document | +-----------------+-----------------------------+-----------------+ | VncUsername | VNC server username | this document | +-----------------+-----------------------------+-----------------+ | VncPassword | VNC server password | this document | +-----------------+-----------------------------+-----------------+ | SecurityType | RFB security type used | this document | +-----------------+-----------------------------+-----------------+ | ChannelType | Connection channel type | this document | +-----------------+-----------------------------+-----------------+ | SshHost | SSH server hostname or IP | this document | +-----------------+-----------------------------+-----------------+ | SshPort | SSH server port | this document | +-----------------+-----------------------------+-----------------+ | SshUsername | SSH username | this document | +-----------------+-----------------------------+-----------------+ | SshPassword | SSH password | this document | +-----------------+-----------------------------+-----------------+ | IdHashAlgorithm | Hash algorithm used with | this document | | | "IdHash" parameter | | +-----------------+-----------------------------+-----------------+ | IdHash | Expected hash of remote | this document | | | public key or certificate | | +-----------------+-----------------------------+-----------------+ | ColorLevel | Client color depth/mode | this document | +-----------------+-----------------------------+-----------------+ | ViewOnly | Client is view only | this document | +-----------------+-----------------------------+-----------------+ | SaveConnection | Store connection info | this document | +-----------------+-----------------------------+-----------------+ Future assignments to this registry should be made through the "First Come First Served" process described in [RFC5226].
5. References
5.1. Normative References
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, DOI 10.17487/RFC1321, April 1992, <http://www.rfc-editor.org/info/rfc1321>. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>. [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003, <http://www.rfc-editor.org/info/rfc3629>. [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005, <http://www.rfc-editor.org/info/rfc3986>. [RFC4251] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251, January 2006, <http://www.rfc-editor.org/info/rfc4251>. [RFC4252] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Authentication Protocol", RFC 4252, DOI 10.17487/RFC4252, January 2006, <http://www.rfc-editor.org/info/rfc4252>. [RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253, January 2006, <http://www.rfc-editor.org/info/rfc4253>. [RFC4254] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Connection Protocol", RFC 4254, DOI 10.17487/RFC4254, January 2006, <http://www.rfc-editor.org/info/rfc4254>. [RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, <http://www.rfc-editor.org/info/rfc5234>. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008, <http://www.rfc-editor.org/info/rfc5246>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, <http://www.rfc-editor.org/info/rfc5280>. [RFC6143] Richardson, T. and J. Levine, "The Remote Framebuffer Protocol", RFC 6143, DOI 10.17487/RFC6143, March 2011, <http://www.rfc-editor.org/info/rfc6143>. [SHS] National Institute of Standards and Technology, "Secure Hash Standard", NIST FIPS PUB 180-4, DOI 10.6028/NIST.FIPS.180-4, August 2015.5.2. Informative References
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008, <http://www.rfc-editor.org/info/rfc5226>. [RFC7595] Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines and Registration Procedures for URI Schemes", BCP 35, RFC 7595, DOI 10.17487/RFC7595, June 2015, <http://www.rfc-editor.org/info/rfc7595>. [X.509] ITU-T, "Information technology - Open Systems Interconnection - The Directory: Public-key and attribute certificate frameworks", ITU-T Recommendation X.509, ISO/IEC 9594-8, 2005.
Appendix A. "vnc" URI Template
This template is provided for registration of the "vnc" URI in the IANA "Uniform Resource Identifier (URI) Schemes" registry as specified in [RFC7595]. Scheme name: vnc Status: Permanent Applications/protocols that use this scheme name: Virtual Network Computing (VNC) remote desktop applications use vnc URIs. VNC applications use the Remote Framebuffer (RFB) protocol. Contact: IESG <iesg@ietf.org>. Change Controller: See the authors of this document. Change control is through the IESG on behalf of the IETF <iesg@ietf.org>. References: This document.
Acknowledgments
Dominic Parkes and the staff of RealVNC Ltd. graciously reviewed this document and provided constructive comments. RFB and VNC are registered trademarks of RealVNC Ltd. in the United States and in other countries.Authors' Addresses
David Warden Dell Products LP 200 Dell Way Round Rock, TX 78682 United States Phone: 512-728-0380 Email: David_Warden@dell.com URI: http://www.dell.com Iordan Iordanov Undatech 260 Scarlet Road, Apt. 503 Toronto, ON M6N 4X6 Canada Email: iiordanov@gmail.com